1. The Field of the Invention
This invention relates to regulating two-phase flow and, more particularly, to novel systems and methods for optimizing production from wells such as those found in oil, gas, and coal bed methane fields.
2. The Background Art
The presence of methane (CH4, a principal ingredient of natural gas) in underground coal seams has long been known. In the past, coal bed methane was vented to provide a non-explosive, non-suffocating environment in which coal miners could work. However, in recent times, methane has become a popular fuel for use in electric generators, furnaces, city buses, and the like. Methane's popularity may largely be attributed to its relatively low cost and clean combustion characteristics. Meanwhile, conventional oil drilling and production is ongoing for petroleum and natural gas.
Various techniques are used to collect coal bed methane. In recent development, water well technology is used to collect methane from coal seam aquifers. By drilling down to a coal seam aquifer and pumping out water, the pressure holding the methane within the coal seam is relieved somewhat as it propels methane and water mixed therewith up the well bore (typically a cased bore). The methane may then be gathered, compressed, and shipped to customers. Well drilling and production techniques permit the collection of methane from coal seams at virtually all depths at which coal is available. Thus, coal bed methane may be collected from coal seams that are far too deep to be mined themselves.
Unfortunately, the best producing coal bed methane wells are generally the most difficult to control and maintain. High production coal bed methane wells have a high occurrence of “gas lock.” Gas lock occurs when a pump lifting water from a coal seam aquifer ingests gas (i.e. methane), rather than water. Such pumps are typically electrically driven, submersible types, often with centrifugal impellers, and thus having non-positive-displacement. Often it is difficult for a pump to rid itself of the gas once ingested. Thus, the ingested gas is trapped inside the pump. The pump's impellers are ineffectual to move the gas out and water cannot get to the impellers.
Gas locked pumps are undesirable for two reasons. First, a gas locking occurs, a pump is initially less efficient at lifting water, and performance quickly decays until the pump lifts no water. As a result, water enters the well from the coal seam aquifer at a rate greater than the pump can extract it. Thus, the well tends to fill with water. High water levels increase the pressure head on the well and less methane is able to escape. The water output and related methane production of the well are greatly reduced, and in some cases are even stopped entirely.
The second problem is that gas locked pumps age quickly. The decrease in water flow through the pump results in a substantial decrease in lubrication and cooling and is associated with and responsible for increased water in mechanical components. Electrical insulation and windings in pump motors can melt down from overheating. Moreover, a pump operating at a comparatively elevated temperature tends to accumulate mineral deposits at a faster rate. Thus, a pump in gas locking situation is much more likely to seize, damaging the pump and motor. Once a pump fails, it must be pulled from the well and a new pump and motor lowered back in. However, the new pump may be just as susceptible to gas lock as the failed pump. Thus, the costly cycle may continue.
Various devices have been applied to solve the gas lock/gas ingestion problem. For example, progressive cavity pumps (helical augers with positive displacement) have been installed in problematic wells. Progressive cavity pumps are much more expensive to purchase and maintain than centrifugal pumps. However, progressive cavity pumps are better able to ingest gas without losing pumping ability. Ingested gas is expelled from the pump along with everything else ingested. Thus, progressive cavity pumps do not gas lock, technically speaking.
That is not to say, however, the repeated ingestion of gas creates no problems in progressive cavity pumps. Without a steady flow of water, progressive cavity pumps may be insufficiently lubricated and cooled. Thus, ingesting gas shortens the life of progressive cavity pumps much as it does the life of centrifugal pumps. In addition, such pumps have rotors operating between stators covered with elastomeric and other polymeric compounds, which materials may fail due to hysteresis. Hysteresis may be thought of as a failure to return elastically to a neutral (initial unstressed) mechanical position. This may sometimes result from inelastic creep, yielding (plasticity), melting, or the like.
Other devices have been introduced to prevent submersible pumps from ingesting gas. For example, shrouds or “gas jackets” have been used. Gas jackets operate on the assumption that gas bubbles in coal bed methane wells will rise. Under this theory, submersible pumps gas lock by inhaling gas bubbles rising past the pump inlet. Typical gas jackets are designed to create a path wherein all fluids must travel downward a selected distance before they may enter the pump inlet. The operational concept is that since gas bubbles will rise, they will not be able to maintain a downward direction all the way down to the inlet. Gas jackets appear to prevent gas lock in some wells, but perform only marginally. Moreover, gas jackets are completely ineffective in many other wells. Gas jackets may be based on a false premise, that gas bubbles always rise sufficiently fast in moving liquid. Two phase flows may actually carry large amounts of entrained gases in a liquid “matrix,” or large amounts of liquid in a gas environment. Flows may be up, down, or horizontal, including combinations thereof.
As a practical matter, gas locking appears to result from collection of upwardly moving gas, but just as often results from collection of downwardly flowing gas entrained in water moving down toward a pump inlet. Thus, gas jackets have not proven predictable or reliable. Moreover, some gas jackets prevent water from accessing and cooling all parts of the pump motor, resulting in poor cooling flows, resultant overheating, and its attendant consequences, including catastrophic failure.
Other devices, operating on this same assumption, that gas bubbles in coal bed methane wells generally rise, have been used. One such device employs a “stinger,” a narrow extension of a shroud extending down below the actual pump inlet to the very lowest point reasonably possible within the well. Theoretically, the lowest point should have the lowest concentration of gas bubbles. The stinger designs may solve the problems of overheating associate with some gas jackets (shrouds) in that water entering the stinger passes over and cools the motor before entering the stages of the pump. However, like other gas jackets and shrouds, these devices prevent gas lock in certain wells, but not all, and not predictably, reliably, or permanently.
Moreover, all such shrouds and gas jackets do not change the fact that a pump inlet is a relative low pressure region to which flows proceed, entraining gas in liquid (water). Conventional oil wells may likewise entrain gases in liquids (petroleum). Some theorize that a certain amount of noncondensible methane will nevertheless be absorbed, and may come out of solution under reduced pressure (pressure less than that at which the gas is absorbed in an equilibrium concentration). At a reduced pressure, the equilibrium concentration of gas changes, releasing gas. Under this theory, the question of gas locking may simply be an issue of flow rate, absorption and desorption rates of gas in water, accumulation, and the pressure differential imposed by the pump inlet in operation. The gas can bubble out of solution as carbon dioxide does from warm or non-pressured soda. By whatever mode, gas for gas locking seems to remain available to interfere with proper pump operation.
What is needed is a simple device and associated method to regulate and control the flow of gas and water in coal bed methane wells, as well as conventional oil wells so that optimum water and other liquid extraction, and thus total methane or oil production, may be maintained.